Optical waveguide light modulator

ABSTRACT

Modulation of light in a fused silica optical waveguide is accomplished by a Faraday effect rotation of the plane of polarization of the light propagating in the waveguide. The fused silica waveguide utilized has unique characteristics of low attenuation and low depolarization due to the purity of the fused silica, the manner in which the fused silica is made, the use of the same base material for the core and cladding layer, and a single mode transmission characteristic. The low attenuation and low depolarization permit low strength magnetic fields to cause large rotations of the plane of polarization of the light beam in the waveguide. Because of the length of the light path exposed to the magnetic field, rotations of at least 90* can be accomplished with the application of as little as 100 oersteds of magnetic field along the longitudinal axis of the guide.

United Stat 1 Borrelli et al.

[111 3,756,690 [451 Sept. 4,1973

[75] Inventors: Nicholas F. Borrelli; Felix P. Kapron, both of Elmira;Donald B. Keck, Big I Flats, all of N.Y.

[73] Assignee: Corning Glass Works, Corning, NY.

[22] Filed: Mar. 30, 1972 [21] Appl. No.: 239,748

[52] US. Cl. 350/151, 350/96 W [51] Int. Cl. G02f l/22 [58] Field ofSearch 350/151, 96 R, 96 B, 350/96 W0 [56] References Cited UNITEDSTATES PATENTS 3,633,992 1/1972 Uchida et a1. 350/151 3,320,114 /1967Schulz 350/96 R 3,030,852 4/1962 Courtney-Pratt... 350/151 3,659,9155/1972 Maurer et al. 350/96 WG 3,660,291 5/1972 Strong 350/151 3,434,7743/1969 Miller t 350/96 WG 3,647,406 3/1972 3,196,739 7/1965 Wenking eta1 350/151 L|GHT'" l2 POLARIZER ll OPTICAL WAVEGUIDE LIGHT MODULATORFIELD COIL, 17

3,605,013 9/1971 Yoshikawa etal. 350/151 Primary Examiner-DavidSchonberg Assistant Examiner-Michael J. Tokar Attorney-Richard E. Kurtzet al.

[57] ABSTRACT Modulation of light in a fused silica optical waveguide,is accomplished by a Faraday effect rotation of the plane ofpolarization of the light propagating in the waveguide. The fused silicawaveguide utilized has unique characteristics of low attenuation and lowdepolarization due to the purity of the fused silica, the manner inwhich the fused silica is made, the use of the same base material forthe core and cladding layer, and

a single mode transmission characteristic. The low attenuation and lowdepolarization pennit low strength magnetic fields to cause largerotations of the plane of polarization of the light beam in thewaveguide. Because of the length of the light path exposed to themagnetic field, rotations of at least can be accomplished with theapplication of as little as oersteds of magnetic field along thelongitudinal axis of the guide.

19 Claims, 6 Drawing Figures AMPLITUDE TIME-+ DETECTOR,23

\ LARIZER, 22

WAVEGUIDE, 15 a OPTICAL WAVEGUIDE LIGHT MODULATOR BACKGROUND OF THEINVENTION This invention relates to optical modulators and moreparticularly the modulation of light transmitted through an opticalwaveguide.

The modulation and gating of light has taken on increasing importance inthe communications field because of the density of information that canbe transmitted on a single beam. One of the methods of controlling andconfining a particular light beam is through what is sometimes referredto as a light pipe. An optical waveguide is one type of light pipe inwhich only selected modes of transmission are permitted. Although thereare various methods of modulating light which is to be transmittedthrough such an optical waveguide, the majority of these modulationtechniques modulate or gate the light prior to its introduction into thewaveguide'so that the only function of the waveguide is to faithfullytransmit the already modulated light from the input of the waveguide toits out put. it is however advantageous and more efficient to be able tomodulate the light as it travels down the optical waveguide so that theoptical waveguide not only serves as a conduit for the light but alsoserves as a modulator or light gate.

The subject invention utilizes the Faraday effect to modulate the lightin the waveguide. The Faraday effect is such that light polarized in agiven direction has its polarization direction changed by theapplication of a magnetic field parallel to a beam of polarized light.This change of polarization direction is called a rotation and is sensedand converted to a change of light beam intensity. The change inintensity of the light beam by the magnetic field is the modulation"referred to herein.

Up until the present, this effect has been observed only in bulk glassand has been of little practical use because of the small rotationswhich could be achieved. The efficient modulation or gating of light bythe application of a magnetic field according to the Faraday effect hasup to now been impossible because of the attenuation both in bulk glassand in prior art light pipes. With high attenuation there cannot be along light path subjected to a magnetic field. In prior art devicesattenuation is on the order of 1000 db/km. However with the subjectwaveguide, attenuation is reduced to db/km or less. Thus a substantialpath of light can be acted upon by a magnetic field when the subjectwaveguide is used. It will be appreciated that in bulk glass of the typeused in observing the Faraday effect, only about minutes of polarizationrotation can be obtained with an applied field of about 100 oersteds fora l cm path length. This small rotation. in bulk glass is due primarilyto attenuation resulting in a comparatively short useable optical pathlength within the glass. It should be noted that even if prior artoptical waveguides were to be used to provide a light path parallel to amagnetic field, attenuation would preclude large Faraday rotations.However, it is now possible with the optical waveguide to be describedto maintain a useable transmission of light through several metersparallel to a magnetic field and to obtain rotations in excess of 90with magnetic fields as small as 100 oersteds.

A second problem with conventional light pipes" is the destruction ofthe initialstate of polarization of an incoming light beam. This iscalled depolarization. In

order for the Faraday effect to be operative, the light beam acted on bythe magnetic field must be either linearly or elliptically polarized. Ineither case there is an identifiable direction of the plane ofpolarization. ln linear polarization the direction of the plane ofpolarization is the direction of the electric field vibration. ln theelliptical case the direction is the direction-of the major axis of theellipse. In prior art light pipes" the initially polarized light beambecomes significantly depolarized" with distance. In general, thelongerthe light pipe the more depolarization that will occur. Thus even if alow attenuation light pipe" is used, Faraday rotations will be obscuredbecause of the depolarization.

Depolarization has two causes. The first is scattering and the second isdue to multi-mode interaction. With respect to scattering, it has beenfound that in the waveguide to be described there is littledepolarization because of the materials used. Depolarization due tomulti-mode interaction can be eliminated by constructing the waveguideto operate in a single mode region. As a result depolarization in thesubject waveguide is less than 1 percent over a distance of l kilometer.The waveguide also has an exceptionally low attenuation because of thepurity of the materials. This polarization preservation quality and lowattenuation makes possible an efficient Faraday efiect opticalmodulator.

As mentioned above in addition to scattering as a cause ofdepolarization there is also a depolarization due to multi-modeinteraction. In general, the larger the number of modes of lightpropagating in the waveguide, the greater will be the depolarization.Normally in a light pipe" there are a very large number of modespropagating. Each of these modes has its own propagation anddistribution characteristics. All of the modes interact with one anotherto vary the phase of the light in the light pipe." This causes the abovedestruction of polarization. However in the subject waveguide, there areonly a few selected modes. These modes can operate independently of oneanother such that the waveguide can be considered a single modewaveguide. When only these modes are present, polarization is preservedthroughout the length of the waveguide. In other words, it is a findingof this invention that optical waveguides supporting only either one ora few modes can be produced which transmit light without degrading thepolarization states of these modes regardless of the length of thewaveguide.

It should be noted that in the prior art light pipes" where a very largenumber of modes are propagated,

the magnetic field operates on all of these modes, re-

sulting in a masking of the Faraday effect. Because only one or a fewmodes are transmitted in the subject waveguide, dilution or intermixingof Faraday rotations is not a problem.

In summary, by use of a unique waveguide, significant rotations can beachieved and a practical modulator can be built. As mentioned, thewaveguide is characterized by an extremely low attenuation and a lowdepolarization due to a number of characteristics of the waveguide. Thefirst characteristic of the waveguide is that both the core and thecladding layer are made of fused silica. It is this material whichenables large rotations with low level magnetic fields. Fused silica cannow be made substantially pure by a flame hydrolysis soot depositiontechnique. This technique produces a raw material so pure that lightscattering is minimal in the finished waveguide. This is importantbecause scattering of any kind results in attenuation and depolarizationof a light beam as it travels down any substantial length of waveguide.As mentioned before, the other portion of the depolarization comes frominteraction between the very large number of modes of light that canpropagate in a light pipe. When fused silica is doped two things occur.First, the index of refraction of the core or the cladding can bereadily changed by selective doping. Secondly the Verdet constant of thefused silica can also be readily altered by doping of the fused silica.If the index of refraction of the core of the waveguide is made onlyslightly larger than that of the cladding, the waveguide operates totransmit only selected modes. By limiting the number of modes that canpropagate, the other portion of the depolarization is eliminated.Further if rare earth or lead dopants are used, the Verdet constant ofthe fused silica can simultaneously be increased. This markedlyincreases the sensitivity of the waveguide to magnetic fields so thatmodulation can be achieved with low level magnetic fields. Finallybecause the cladding layer and the core are made of the same basematerial, mechanical stability is increased because of thermal andphysical matching. It can be seen therefore that low attenuation and lowdepolarization are made possible by use of fused silica as the waveguidematerial. These two characteristics pennit the use of long waveguideswhich can maintain a significant optical path length parallel to amagnetic field. This in turn results in rotations exceeding 90 for a 100oersted field and a path length of 54 meters instead of only 30 minutesusing a 100 oersted field and a usable 1 cm path length bulk glass.

SUMMARY OF THE INVENTION It is therefore an object of this invention toprovide an improved optical modulator and an improved method of lightmodulation.

It is a further object of this invention to provide in combination anoptical waveguide having low attenuation and'low depolarization and aFaraday type light modulation system so as to modulate or gate the lightas it passes through the waveguide.

In accordance with an important aspect of this invention, the modulatorconsists essentially of an input light beam polarized in a givendirection, an optical waveguide of the type described adapted totransmit the polarized light beam, means for developing a modulatedmagnetic field along the longitudinal axis of the waveguide and anoutput polarizer.

Selective application of the magnetic field as well as variation of themagnetic field causes a modulation of the light beam traveling throughthe waveguide by rotating its plane of polarization. In this manner, theamplitude or intensity of the light beam can be efficiently modulated oreven gated by changing the amount of rotation induced by the externalmagnetic BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagramof .the subject light modulation system showing an incoming polarizedlight beam passing through an optical waveguide and thence through afurther polarizer to a detector with means for selectively providing amagnetic field in longitudinal direction about the waveguide;

FIG. 2 shows an alternate embodiment of the light modulator in which thewaveguide is folded into a toroidal configuration so as to present anincreased length of waveguide to a magnetic field produced by a fieldcoil surrounding the waveguide;

FIG. 3 is a schematic diagram indicating another method of folding theoptical waveguide so as to present an increased length of waveguide to amagnetic field induced by the field coil shown;

FIG. 4 is a representation in cut-away of one method of forming thesubject single-mode waveguide indicating a drawing technique;-

FIG. 5 is a cross-section of the structure shown in FIG. 4 taken alongthe 5-5 axis; and

FIG. 6 is a cross-section of the structure shown in FIG. 4 taken alongthe 6-6 axis indicating the completed waveguide structure cross-section.

DETAILED DESCRIPTION OF THE INVENTION The subject light modulatorinvolves the combination of a unique optical waveguide and certain meansfor inducing a Faraday rotation of the plane of polarization of lighttraveling through the waveguide. The Faraday effect described herein isthe rotation of the plane of polarization of a light beam by the actionof a magnetic field along the direction of the light beam. The usualexpression for the effect is given by the following formula:

0 (min of arc) VH(oersteds) X L(cm) where V is the Verdet constant and His the field in the direction of the light beam.

If an isotropic optical waveguide fiber is enclosed in a solenoid andcoherent polarized light is sent into the guide, the action of themagnetic field will rotate its plane of polarization by an amount givenby the above equation where L" is the length of the fiber enclosed bythe solenoid. If the output is incident on a polarizer rotated from theinitial polarized light direction then upon application of the field,light will come through to the extent given by the expression I/I, sin 0sin (VI-IL).

In the event of a birefringent fiber as for example produced by anelliptical core, the device can be operated in the above mode byinputting the polarized light beam along the fast" or slow" axis of thewaveguide or by the insertion of a compensating phase plate. The purposeof the phase plate is to remove a DC component of the light output ofthe waveguide which is introduced by elliptical polarization. It will beappreciated that elliptical polarization can occur due to birefringencein the waveguide. However, for either case the arrangement shown in FIG.1 will efficiently cause a light modulation.

a. The Modulator In FIG. 1, coherent light beam 10 is made incident on afirst polarizer 11 which polarizes the light in a direction indicated bythe arrow 12. In one embodiment a coherent light source is used which isa helium-neon laser. Alternately, a non-coher nt light source can beused. Although there will be certain coupling problems with anon-coherent light source, it can be used with reduced efficiency. Thepolarized light is then made incident on the end or input face of awaveguide which is positioned within a solenoid 16 having a field coil17 maintained in position about the waveguide 15. The field coil 17 isenergized by a voltage source 18 and a switch 19 so as to produce amagnetic field, H, in a given direction. The result of the applicationof the magnetic field, H, is the rotation of the plane of polarizationof the light in the waveguide, shown by the arrow 20 to be the angle 0where 0 VI-IL as given hereinbefore. The light beam is then passedthrough a second polarizer 22 whose plane of polarization is rotated 90from that of polarizer 11. Unless the original plane of polarization isrotated no light is transmitted through polarizer 22 to a detector 23.However, when the magnetic field is applied across the waveguide 15 acertain component of the light traveling through the polarizer 22 willnot be cancelled and the detector 23 will register some value. Thisvalue can be computed from the expression [/1 sin 0 sin (VI-IL). In oneoperative embodiment of the invention with a fused silica waveguide theVerdet constant V is 0.01 min/oersted-cm. With a field of 100 oerstedsand a waveguide length of 27 meters, 100 percent modulation is obtainedwith the output polarizer making an angle of 45 with the inputpolarizer.

In another configuration the detector 23 is not used and the modulatorserves as the Q switching element for a laser. In this embodiment eitherno light is transmitted or there is a full transmission of light. Thisgating effect is due to the ability to rotate the plane of polarizationby 90 with small magnetic fields. For purposes of this invention theword modulation is taken to include gating" as just described. It willbe appreciated that the selective application of the voltage 18 throughthe solenoid coil produces a modulation which can be in pulse form asshown by the graph to the right of the detector 23. The replacing of theswitch 19 by any type of modulation system will produce analogousrotations of the light within the waveguide 15 and therefore analogousamplitude variations of the light beam at the detector 23.

It will be appreciated that the longer the waveguide in the magneticfield, the greater the rotation due to the magnetic field. Long lightpaths can be easily achieved by sending the fiber back and forth throughthe solenoid. This is called folding the light path and is possibleinsofar as the sense of the Faraday rotation is independent of lightdirection. With such a folding of the light path significant rotationscan be obtained with relatively low magnetic field strengths. The foldedconfigurations are shown in FIGS. 2 and 3 in which like elements arelabeled with like reference characters. From FIG. 2 it can be seen thatthe most practical device would involve a toroidal configuration withthe waveguide 15 encircled by a solenoid of many turns.

A linearly folded waveguide as shown in FIG. 3 includes a coherent lightsource 25. A driver 26 takes the place of elements 18 and 19. The drivermay be any type of voltage source which can be modulated including an ACsource upon which is impressed a certain DC level. In this case it isimportant to note that the sense of the Faraday rotation is in factindependent of the propagation direction such that linear folding doesnot result in a cancellation of the magnetically induced rotation.

In the event that linearity of amplitude is required between the outputlight and the impressed signal, a bias may be applied either by aconstant background H field, or where possible by rotation of the outputpolarizer off the null position. If the output polarizer is shifted offnull by an amount 0, then the output is given by the expression Ill, sin(VHIrO It will be appreciated that a single mode waveguide can be madeto have a core and a cladding member. In this invention, the core andthe cladding members are made of very similar materials such that theindex of refraction of the core is not much greater than the index ofrefraction of the cladding. This permits a single mode or a limitednumber of modes of light to be transmitted. Several embodiments of thesubject single-mode" waveguide are possible. In one embodiment of theinvention a cladding member of pure fused silica is combined with a corecomposed of fused silica containing a suitable amount of one or moredoping materials. The dopant is incorporated in the material of the corein an amount calculated to provide an increase in the refractive indexrequired for particular waveguide applications. Reference to a dopant isintended to include both individual additives and combinations ormixtures.

In a second embodiment, a cladding member and core are both doped fusedsilicas and a dopant is the same in each material. In this embodiment,the core is necessarily more heavily doped; that is, provided with asuitably larger amount of dopant in the core and cladding member iscalculated to provide the refractive index differential required forwaveguide purposes.

Finally, the cladding member and the core may both be composed of dopedfused silicas wherein different dopant or combinations thereof are usedin the two parts. In this situation, the amount or portion of dopant inthe core may or may not be made greater than that in the cladding. Thus,the dopant in the cladding may have a relatively small effect onrefractive index in the glass as compared with the dopant in the core.In that event, the actual weight of dopant in the cladding may be thatin the core. The significant factor is the actual refractive indexdifferential, and this must be positive in the direction of the core.Thus, agiven amount of dopant must provide more of an increase in therefractive index of the core than in the cladding member, regardless oftheir respective amounts.

In the absence of other factors, the first embodiment is the mostconvenient to produce and hence preferred. However, where close matchesof physical properties such as annealing temperature or expansioncoefficient are important, and where other secondary factors becomeimportant, the second or third embodiment may prove desirable.

It will be appreciated, however, that any low loss and- /or polarizationpreserving optical waveguide is included as part of this inventionwhether or not it is made of fused silica.

b. The Waveguide This invention utilizes a completely new and novel typeof material for the production of optical waveguides and recognizesparticular properties of this novel waveguide which make it uniquelyuseful in modulating or gating previously polarized light travelingthrough the waveguide. Heretofore, less refractory and fibercharacteristic term easily workable materials have been used in theproduction of optical waveguides. It has now been discovcred thatsubstantially pure fused silica which is extremely hard and difficult towork, can beused to produce an optical waveguide in which light can bereadily modulated. The term pure fused silica is used herein to indicatea fused silica containing no elemental impurities in an amount greaterthan 0.l percent by weight except for hydrogen which may be present inamounts up to percent by weight. As mentioned hereinbefore, it was foundthat not only can such a fused silica optical waveguide be providedwhich selectively transmits only a single or a few modes of light, butalso the polarization state of any light passing therethrough ispreserved through the waveguide. This permits an extremely efficientFaraday rotation to be imparted to light traveling within the waveguide.

The selective transmission of certain modes in an optical waveguide ismade possible by appropriate indices of refraction of the core andcladding layer. The condition under which propagation of a particularmode will no longer be localized about the core of an optical fiber canbe expressed in terms of a cut-off value U. An exceptionally complexequation, and an explanation thereof, from whichthe value U for aparticular mode can be determined may be found on page 55 of FiberOptics Principles and Applications, by N. S. Kapany.

On the same page of this book Kapany expresses a R in terms of theoptical fiber variable by the equation R mm ("1* m) where a core radiusof the waveguide A wavelength of light to be transmitted n core index ofrefraction n, cladding index of refraction which can be rewritten as R(21ra/X) ("t 1) ("i "O Then as is explained in Kapany, for a particularmode to propagate within an optical fiber having a particular "fibercharacteristic term R," "R" must be greater than or equal to the cutoffvalue U for the mode.

For instance the mode HE the definition and physical characteristics ofwhich can be found in the aforementioned cited sources, is the only modeof light that will propagate along a fiber which has an R value of lessthan 2.405. Therefore, if R is set equal to 2.405 and equation (3) isevaluated, it can be seen that a method of limiting light propagation ofa desired wavelength to one mode is to coordinate the parameters a, n,and n, of the waveguide. That is, if the difference between the twoindices of refraction (n,-n,) increases, the core radius 4 mustdecrease; and if (n,n,) decreases, the core radius a must increase.

It is however difficult to produce a waveguide having core and claddingindices of refraction within limits necessary to maintain single modepropagation when the waveguide has a very small core. The difficultyincreases markedly in producing waveguides with larger 1 cores. Forexample, an optical waveguide having a small core diameter ofapproximately one micron requires a difference in the two indices ofrefraction on the order of IO". However, if the optical waveguide has alarge core diameter of approximately one millimeter, a minutedifferential between the two indices of refraction of approximately 10*is required. This large core is made possible by a flame hydrolysismethod to be described hereinafter.

By analogy waveguides having a limited" number of modes propagating canalso be produced by suitable doping. An example of one set of parametersfor a waveguide capable of propagating only the first seven modes isshown in the following table:

TABLE I Cladding Composition: l00% SiO, Core Composition: 5.25 wt.% TiO,94.75 wt.% SiO, Light Wavelength: A 5893 A Core Diameter: d =-6p.m

The difiiculty of very accurately controlling the diameter of the core,while maintaining a small, but precise differential between the index ofrefraction of the core and the index of refraction of the claddinglayer, has, in the past, made the production of optical waveguides avery slow and expensive process. However by using fused silica for boththe core and the cladding, and by appropriate dopings, single modewaveguides can be easily fabricated. The fused silica waveguide is thesubject of copending applications entitled Method of Producing OpticalWaveguide Fibers" by Donald B. Keck and Peter C. Schultz, Ser. No.36,267, filed May 1 1, i970 (Keck-Schultz, 1-2) and Glass OpticalWaveguide by Robert D. Maurer and Peter C. Schultz, Ser. No. 36,109,filed May ll, 1970 (Maurer- Fused silica is particularly suitable forproduction of waveguides in accordance with the requirements set forthabove because fused silica is readily attainable with exceptionally highpurity, and because such pure forms have a very precise reproducableindex of refraction. By adding a precise percentage by weight of adoping material to the fused silica a doped fused silica" with an indexof refraction that is higher than that of pure fused silica by a precisereproducable amount is readily produced.

Because the amount of doping material necessary to give the desiredincrease in the index of refraction of fused silica is small, thephysical characteristics, such as viscosity, softening point andcoefficient of expansion of the fused silica in-the core can be madealmost identical to those of the fused silica used for the claddingmember. This substantially eliminates some of the difficultiesencountered in drawing waveguides, such as excess internal strain, andlarge viscosity differential. Accordingly, it is generally desirablethat the doping mat erial not exceed 15% by weight of the glass.

in addition, pure fused silica has excellent light transmissionqualities since both absorption of light energy and intrinsic scatteringof light are exceptionally low in such material. It is essentially freeof the transition metal oxides and other light absorbing or glasscoloring materials. Scattering of light caused by the presence ofparticulate impurities rather than the intrinsic nature of the materialitself is low. It will be understood, of course, that the dopants addedto fused silica in accordance with the present invention neither absorbnor scatter light energy to any anpreciable extent in the wavelength ofinterest. Further, fused silica is a material such that an opticalwaveguide once fonned possesses the quality of being highly resistant todamage from high temperatures, corrosive atmosphere, and other severeenvironments.

Basically, the preferred waveguide consists of a fused silica claddingand a fused silica core doped to give the waveguide a single modeorlimited mode characteristic. The purity of the fused silica as well asits doping comes from a flame hydrolysis soot deposition followed by adrawing procedure as outlined below.

0. Methods of Making the Waveguide After fused silica blanks or tubesare made available from the flame hydrolysis method referred to in theaforementioned copending application of Keck and Schultz, and describedhereinafter, the waveguide possessing a pure fused silica cladding and adoped fused silica core may be produced by any suitable method includingbut not limited to (a) insertion of a rod of fused silica, doped asrequired to increase the index of refraction to the desired level abovethat of pure fused silica, into a tube of pure fused silica; raising thetemperature of the rod and tube combination until the combination has alow enough viscosity for drawing; and then drawing the rod and tubeuntil the tube collapses around and fuses to the rod, such that thecorresponding cross-sectional area of combination is decreased to thedesired dimension; or (b) the method comprising first forming a film ofpre-doped silica on the inside wall of the tube of pure fused silica;and then drawing this composite structure to produce a crosssectionalarea resulting fromthe collapse of the film of doped fused silica tofonn a fiber having a solid crosssection of the desired diameter.

There are many dopant materials which can be added to fused silica inminute quantities to increase its index of refraction to a predeterminedlevel. In order to raise the Verdet constants, rare earth dopants arepreferred. However, a primary factor to consider is the diffusionproperty of the dopant material.

When producing one type of optical waveguide, dopant diffusion is anecessity. Here, the core is drawn with a small diameter and subsequentdiffusion increases the core diameter to the desired size. Dopingmaterials containing alkali ions are readily diffused into the fusedsilica cladding of an optical waveguide and increase the effective corediameter.

Most waveguides, however, require a dopant that does not diffuse withinthe cladding to any appreciable extent. Specifically, the dopant mustnot diffuse out of the core and into the cladding member either duringproduction or during subsequent service. Such diffusion wouldeffectively increase the diameter of the core and thereby alter the modeselection abilities of the waveguide. Suitable dopants having minimumdiffusion properties include the multivalent metal oxides: titaniumoxide, tantalum oxide, tin oxide, lead oxide, niobium oxide, zirconiumoxide, and aluminum oxide and the rare earth oxides: ytterbium oxide,lanthanum oxide, terbium oxide, dysprosium oxide and praseodymium oxide.

The addition of these oxides, individually or in combination to a fusedsilica glass raises the refractive index of such a glass in apredictable manner. In general, a linear relationship exists between theamount of oxide added and both the absolute refractive index anddifferential or increase. Again those dopants raising the Verdetconstant of the fused silica are the rare earth oxides and lead oxides.

It may be noted that when the weight percent of the oxide is plottedagainst the refractive index there is a break in both the TiO, and theTa,0,, curves. This is believed to be associated with structural changesin the glass. in any event a desired refractive index differential orincrease can be calculated in terms of weight percent addition for anydesired oxide.

As shown in FIGS. 4-6, production method (b), as described more fully inthe Keck-Schultz application, involves applying a doped silica film, 40,on the inside wall of a fused silica tube 41, then heating the tube bysuitable means 50 to vitrify the doped silica film and soften the filmedtube for drawing into a solid fiber 42 with the film forming a centralcore 43. In carrying out this method, the fused silica to be used as thecore member is produced in the following manner. A suitably proportionedmixture of vapors of hydrolyzable compounds of silicon and a dopantelement are passed through a burner to produce a corresponding oxidesoot by flame hydrolysis. This soot is forced into the cladding layertube and deposited on the inside surface thereof. When a film ofadequate thickness is deposited, the soot generation is stopped andthefilmed tube is heated to vitrify the film, soften the tube, and collapsethe composite into rod form. The softened rod may then be drawn down todesired size.

It has been found that light transmission qualities may be improved indoped fused silica formed into optical waveguides, if the waveguides aredrawn in an oxygen atmosphere and then "heat treated" in an oxygenatmosphere. The heat treatment" referred to consists of heating thewaveguide in an oxygen atmosphere to between 500C and l000C, for notless than thirty minutes; the length of treatment being related to thetreatment temperature. Lower temperatures require longer treatmentperiods, while treatment at the higher temperature allows shorter timeperiods,

A specific example of a waveguide produced by the practice of thisinvention follows. A l k to 2 micron film of fused silica doped withtitanium was bonded to the inside wall of a inch outside diameter, 56inch inside diameter, substantially pure fused silica tube. Prior to thedrawing process, a nominal 3.00 percent of dopant material is added. Asthis diffuses into the cladding,

the effective concentration of dopant is reduced. The

deposited doped fused silica essentially consisted of 97 percent fusedsilica and a nominal 3 percent titanium dioxide. The composite structurewas then heated in substantially an oxygen atmosphere until it reached atemperature at which the materials had low enough viscosity for drawing(approximately 1900C). The composite structure was then drawn to reducethe diameter thereof until the film of titanium doped fused silicacollapsed to seal the longitudinal holeand form a solid core surroundedby pure fused silica. The resulting composite rod was then further drawnto reduce the diameter. thereof to a final diameter of approximately 100microns. The core of the optical waveguide was measured at approximately3 microns in diameter. The core index of refraction was approximately1.466 while the cladding had an index of refraction of approximately1.4584. After the fiber was drawn, it was heat treated at 800C in anoxygen atmosphere for approximately three hours.

In carrying out production method (a) mentioned earlier, a glass tubeand a glass rod are first formed separately and then combined into acomposite article. For this method of production, the glasses of theinvention may be produced by normal glass melting procedure; that is, bymixing a suitably proportioned batch of silica and the dopant oxide, andfusing the batch mixture. Alternatively, a soot technique, as describedin U.S. Pat. No. 2,326,059, granted Aug. 3, 1943 to Dr. H. E. Nordberg,or a gelation technique, followed by fusion, may be employed.

d. The Preferred Process for Making Fused Silica Flame HydrolysisHowever, the glasses of the invention tend to be highly refractoryglasses. Therefore, it is difficult to attain a sufficiently homogenousmaterial by the above batch processing techniques to insure an adequatedegree of optical quality for waveguides. Accordingly, to provide forthe exceptionally high purity, the flame hydrolysis technique describedin detail in the Nordberg patent mentioned above may be employed.

In this procedure, a suitably proportioned mixture of vapors ofhydrolyzable compounds of silicon and the dopant element is entrained ina dry carrier gas such as oxygen. The vapor mixture is passed through aburner to hydrolyze the vapors and convert them to the correspondingoxides by flame hydrolysis. The oxides thus formed melt in the flame andare deposited in vitreous form on a suitable mandrel or bait. In thismanner, a substantial deposit of fused silica is built up in a formcommonly known as a boule. V

The boule may be removed from the deposition furnace and rapidly cooledto room temperature for inspection purposes if desired. Alternatively,it may be transferred to a furnace at or slightly above the fused silicaannealing temperature and then cooled at a suitable rate to provide anannealed body. Finally, the boule may be cut to desired shapes andreworked and- /or redrawn as described earlier.

in carrying out this modified flame hydrolysis method, any hydrolyzablecompound of silicon and of the desired dopant may be used. In the eventthe compounds are compatible in mixture and have suitable vaporpressures, a liquid mixture may be used. Otherwise, the compounds may bevaporized separately and the vapors combined in suitable proportion. lfnecessary, the delivery system is kept properly heated to avoidcondensation.

It is generally preferrable to use the metal chlorides as thehydrolyzable compounds. These are the least expensive and most readilyavailable. Also they are convenient to use and produce by-products thatare easier to control. However, from a technical standpoint, any othervaporizable and hydrolyzable compound may be substituted. These includeparticularly the other halides and organo-metal compounds.

The vapors may be entrained by any carrier gas that does not react withthe vapor during entrainment. This includes inert gases such asnitrogen, and combustible gases such as oxygen and natural gas. It isdesirable to use an oxygas burner for the flame hydrolysis step, andthen employ either oxygen or natural gas as the carrier gas. The carriergas must, of course, be dry to avoid premature hydrolysis of the vaporsand consequent clogging of the delivery system.

e. The Preferred Waveguide The preferred waveguide has both core andcladding made of fused silica or doped fused silica. The purity of thefused silica is obtained through a flame hydrolysis method. Doping isachieved by adding rare earth or lead dopants at the time the fusedsilica is formed. These dopants raise the Verdet constant of thewaveguide. The dopant concentration in the core and cladding are suchthat a single mode transmission characteristic is achieved. When such awaveguide is used in the system described, magnetic fields on the orderof 100 oersteds provide rotation greater than for waveguides havinglengths of 54 meters.

What is claimed is:

1. An optical modulator for modulating a beam of 7 light initiallypolarized in a predetermined direction comprising in combination,

an optical waveguide, composed of a core member and a cladding membersurrounding said core, the index of refraction of said core member beinggreater than the index of refraction of said cladding member by anamount such that a limited number of modes of said light are transmittedby said waveguide, said waveguide being of a purity and of aconfiguration so as to transmit without substantial loss over asubstantial length thereof the limited number of modes of light, saidwaveguide being such that the polarization of any light beam in saidwaveguide remains substantially preserved during transmission from aninput face to an output face of said waveguide, and

means for generating a modulated magnetic field along thelongitudinalaxis of saidwaveguide such that the initial plane ofpolarization of any beam of light in said waveguide is rotated aboutsaid longitudinal axis by an amount proportional to the strength of saidmagnetic field, whereby the rotation of the direction of polarization ofthe light in said beam by said magnetic field constitutes modulation ofsaid light beam.

2. The optical modulator as recited in claim 1 and further including incombination therewith;

means for polarizing light, said means being adjacent to said outputface and oriented to pass only light from said output face having apolarization direction at an angle difierent from said initialpredetermined direction, whereby light emanating from said output facepassing through said means for polarizing is modulated in amplitude inaccordance with modulation of said magnetic field.

3. The optical modulator as recited in claim 1 wherein said waveguide isfolded with the folded sections having longitudinal axes parallel tosaid magnetic field.

4. The optical waveguide as recited in claim 3 wherein said waveguide istoroidal in form.

5. The optical modulator as recited in claim 1 wherein said core memberand said cladding member are basically made of fused silica doped sothat the index of refraction of said core member is greater than saidcladding member.

6. The optical modulator as recited in claim 5 wherein said fused silicamembers are prepared in a flame hydrolysis process to obtainsubstantially pure fused silica, whereby said waveguide achievesexceptionally low attenuation and exceptionally low depolarization.

7. The optical modulator as recited in claim wherein one of said membersis fused silica doped with at least one 'or more oxides selected fromthe group consisting of titanium oxide, tantalum oxide, tin oxide,niobium oxide, zirconium oxide, aluminum oxide, lead oxide, and the rareearth oxides.

8. The optical modulator as recited in claim 5 wherein one of saidmembers is fused silica doped with at least one material selected fromthe group consisting of lead and the rare earth elements, whereby theVerdet constant of said waveguide is increased thereby increasing theefficiency of said modulator.

9. The optical modulator as recited in claim 5 wherein said claddingmember is fused silica doped with at least one multivalent metal oxideselected from the group consisting of titanium oxide, tantalum oxide,tin oxide, niobium oxide, zirconium oxide, aluminum oxide, lead oxide,and the rare earth oxides.

10. The optical modulator as recited in claim 5 wherein said claddingmember is undoped fused silica and the core is fused silica containing adopant which provides a precise positive differential between therefractive index of said core member and that of the cladding member.

11. The optical modulator as recited in claim wherein said core memberconsists of fused silica doped with oxides selected from the groupconsisting of lead oxides and the rare earth oxides whereby the Verdetconstant of said waveguide is increased.

12. The optical modulator as recited in claim 11 wherein said coremember is formed of 97 percent by weight of fused silica and a nominal 3percent titanium dioxide.

13. The optical modulator as recited in claim 10 wherein said coremember is formed of at least 85 percent by weight of fused silica and aneffective amount up to 15 percent by weight of doping material.

14. The optical modulator as recited in claim 5 wherein said claddinglayer and said core member are both composed of fused silica'containingthe same dopant, the proportion of the dopant in the fused silica ofsaid core member exceeding that in the fused silica of said claddingmember by a determined amount to provide a precise positive differentialbetween the refractive index of said core member and that of said clad-17. An optical modulator for modulating a coherent beam of lightinitially polarized in a predetermined direction comprising incombination,

an optical waveguide made of fused silica having a purity whichtransmits without substantial loss over a substantial length of saidwaveguide a limited number of modes of light, and means for generating amodulated magnetic field along the longitudinal axis of said waveguidesuch that the initial plane of polarization of any polarized beam oflight introduced into said waveguide is rotated about said longitudinalaxis by an amount proportional to the length of said magnetic field suchthat the direction of polarization of the light in said beam is modifiedin accordance with the modulation of said magnetic field as said lighttravels down said optical waveguide.

18. The optical modulator as recited in claim 17 wherein said fusedsilica is made by flame hydrolysis, which produces a high puritywaveguide material, said high purity waveguide material reducingattenuation and depolarization to such an extent that polarizationrotations in said waveguide are possible with magnetic fields as low asoersteds.

'19. The optical waveguide as recited in claim 18 wherein said waveguideincludes a core member and a cladding member of fused silica, one ofsaid members being doped so as to limit the number of modes saidwaveguide will transmit whereby depolarization of said light beam islimited.

-# I t i t

2. The optical modulator as recited in claim 1 and further including incombination therewith; means for polarizing light, said means beingadjacent to said output face and oriented to pass only light from saidoutput face having a polarization direction at an angle different fromsaid initial predetermined direction, whereby light emanating from saidoutput face passing through said means for polarizing is modulated inamplitude in accordance with modulation of said magnetic field.
 3. Theoptical modulator as recited in claim 1 wherein said waveguide is foldedwith the folded sections having longitudinal axes parallel to saidmagnetic field.
 4. The optical waveguide as recited in claim 3 whereinsaid waveguide is toroidal in form.
 5. The optical modulator as recitedin claim 1 wherein said core member and said cladding member arebasically made of fused silica doped so that the index of refraction ofsaid core member is greater than said cladding member.
 6. The opticalmodulator as recited in claim 5 wherein said fused silica members areprepared in a flame hydrolysis process to obtain substantially purefused silica, whereby said waveguide achieves exceptionally lowattenuation and exceptionally low depolarization.
 7. The opticalmodulator as recited in claim 5 wherein one of said members is fusedsilica doped with at least one or more oxides selected from the groupconsisting of titanium oxide, tantalum oxide, tin oxide, niobium oxide,zirconium oxide, aluminum oxide, lead oxide, and the rare earth oxides.8. The optical modulator as recited in claim 5 wherein one of saidmembers is fused silica doped with at least one material selected fromthe group consisting of leAd and the rare earth elements, whereby theVerdet constant of said waveguide is increased thereby increasing theefficiency of said modulator.
 9. The optical modulator as recited inclaim 5 wherein said cladding member is fused silica doped with at leastone multivalent metal oxide selected from the group consisting oftitanium oxide, tantalum oxide, tin oxide, niobium oxide, zirconiumoxide, aluminum oxide, lead oxide, and the rare earth oxides.
 10. Theoptical modulator as recited in claim 5 wherein said cladding member isundoped fused silica and the core is fused silica containing a dopantwhich provides a precise positive differential between the refractiveindex of said core member and that of the cladding member.
 11. Theoptical modulator as recited in claim 10 wherein said core memberconsists of fused silica doped with oxides selected from the groupconsisting of lead oxides and the rare earth oxides whereby the Verdetconstant of said waveguide is increased.
 12. The optical modulator asrecited in claim 11 wherein said core member is formed of 97 percent byweight of fused silica and a nominal 3 percent titanium dioxide.
 13. Theoptical modulator as recited in claim 10 wherein said core member isformed of at least 85 percent by weight of fused silica and an effectiveamount up to 15 percent by weight of doping material.
 14. The opticalmodulator as recited in claim 5 wherein said cladding layer and saidcore member are both composed of fused silica containing the samedopant, the proportion of the dopant in the fused silica of said coremember exceeding that in the fused silica of said cladding member by adetermined amount to provide a precise positive differential between therefractive index of said core member and that of said cladding member.15. The optical modulator as recited in claim 5 wherein both saidcladding member and said core member consist of fused silica doped withtitanium oxide.
 16. The optical modulator as recited in claim 5 whereinsaid cladding member and said core member are both composed of fusedsilica containing a dopant, the dopant in the core being different fromthat in the cladding member and being present in an amount sufficient toprovide a precise positive differential between the refractive index ofthe core and that of the cladding.
 17. An optical modulator formodulating a coherent beam of light initially polarized in apredetermined direction comprising in combination, an optical waveguidemade of fused silica having a purity which transmits without substantialloss over a substantial length of said waveguide a limited number ofmodes of light, and means for generating a modulated magnetic fieldalong the longitudinal axis of said waveguide such that the initialplane of polarization of any polarized beam of light introduced intosaid waveguide is rotated about said longitudinal axis by an amountproportional to the length of said magnetic field such that thedirection of polarization of the light in said beam is modified inaccordance with the modulation of said magnetic field as said lighttravels down said optical waveguide.
 18. The optical modulator asrecited in claim 17 wherein said fused silica is made by flamehydrolysis, which produces a high purity waveguide material, said highpurity waveguide material reducing attenuation and depolarization tosuch an extent that polarization rotations in said waveguide arepossible with magnetic fields as low as 100 oersteds.
 19. The opticalwaveguide as recited in claim 18 wherein said waveguide includes a coremember and a cladding member of fused silica, one of said members beingdoped so as to limit the number of modes said waveguide will transmitwhereby depolarization of said light beam is limited.